The present invention relates to the manufacture of substrates. More particularly, the invention provides a technique for cleaving a thin film from a substrate that is reusable. The thin film can be used in the fabrication of a silicon-on-insulator substrate for semiconductor integrated circuits, for example. But it will be recognized that the invention has a wider range of applicability; it can also be applied to other substrates for multi-layered integrated circuit devices, three-dimensional packaging of integrated semiconductor devices, photonic devices, piezoelectronic devices, microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d), sensors, actuators, solar cells, flat panel displays (e.g., LCD, AMLCD), biological and biomedical devices, and the like.
Wafers for electronic device fabrication are often cut from an ingot, or boule, of material with an abrasive saw. The wafer often serves as both a mechanical substrate and a semiconductor material to form electronic devices in or on. One of the most common examples of this is cutting silicon wafers from a silicon ingot. The wafers are typically polished to a very fine surface finish after removing the mechanical damage left by the abrasive saw. In some processes, devices are fabricated directly in or on the silicon wafer. In other processes, a layer of semiconductor material is grown, for example by epitaxy, on the wafer. An epitaxial layer may provide lower impurity concentrations, or be of a different semiconductor type than the wafer. The devices are formed in what is known as the xe2x80x9cactivexe2x80x9d layer, which is typically only a micron or so thick.
Sawing wafers from an ingot has several disadvantages. First, a significant amount of material may be lost due to the width, or kerf, of the saw blade. Second, the wafers must be cut thick enough to survive a typical circuit fabrication process. As the wafers get larger and larger, the required thickness to maintain sufficient strength to be compatible with given wafer handling methods increases. Third, the polishing process to remove the saw marks takes longer and removes yet more precious material than would be required if an alternative method existed.
The desire to conserve material lost to the sawing and polishing operations increases as the value of an ingot increases. Single-crystal silicon ingots are now being produced with diameters of twelve inches. Each wafer cut and polished from these ingots can cost over a thousand dollars. Ingots of other materials are also being produced. Some of these materials may be difficult to produce as a single crystal, or may require very rare and expensive starting materials, or consume a significant amount of energy to produce. Using such valuable material to provide simple mechanical support for the thin active layer is very undesirable, as is losing material to the sawing and polishing operations.
Several materials are processed by cleaving, rather than sawing. Examples include scribing and breaking a piece of glass, or cleaving a diamond with a chisel and mallet. A crack propagates through the material at the desired location to separate one portion of material from another. Cleaving is especially attractive to separate materials that are difficult to saw, for example, very hard materials. Although the cleaving techniques described above are satisfactory, for the most part, as applied to cutting diamonds or household glass, they have severe limitations in the fabrication of semiconductor substrates. For instance, the above techniques are often xe2x80x9croughxe2x80x9d and cannot be used with great precision in fabrication of the thin layers desired for device fabrication, or the like.
From the above, it is seen that a technique for separating a thin film of material from a substrate which is cost effective and efficient is often desirable.
According to the present invention, a technique for removing thin films of material from a reusable substrate is provided. This technique separates thin films of material from a donor substrate by implanting particles, such as hydrogen ions, into the donor substrate, and then separating the thin film of material above the layer of implanted particles. A second implant and separation process is then performed to remove multiple films from a single substrate.
In a specific embodiment, the present invention provides a process for forming a film of material from a donor substrate, which is reusable, using a controlled cleaving process. That process includes a step of introducing energetic particles (e.g., charged or neutral molecules, atoms, or electrons having sufficient kinetic energy) through a surface of a donor substrate to a selected depth underneath the surface, where the particles are at a relatively high concentration to define a thickness of donor substrate material (e.g., thin film of detachable material) above the selected depth. To cleave the donor substrate material, the method provides energy to a selected region of the donor substrate to initiate a controlled cleaving action in the donor substrate, whereupon the cleaving action is made using a propagating cleave front(s) to free the donor material from a remaining portion of the donor substrate. The remaining portion of the donor substrate is reused in another cleaving process, if desired.
In another embodiment, a layer of microbubbles is formed at a selected depth in the substrate. The substrate is globally heated and pressure in the bubbles eventually shatters the substrate material generally in the plane of the microbubbles.
The present invention separates several thins films of material from a single, reusable donor substrate. The thin films can be used for fabrication of, for example, a silicon-on-insulator or silicon-on-silicon wafer. A planarizing layer of silicon oxide may be formed on the donor substrate after each cleaving step to facilitate bonding the donor wafer to a transfer wafer, or stiffener. Accordingly, the present invention provides a reusable substrate, thereby saving costs and reduces the amount of scrap material.
In most of the embodiments, a cleave is initiated by subjecting the material with sufficient energy to fracture the material in one region, causing a cleave front, without uncontrolled shattering or cracking. The cleave front formation energy (Ec) must often be made lower than the bulk material fracture energy (Emat) at each region to avoid shattering or cracking the material. The directional energy impulse vector in diamond cutting or the scribe line in glass cutting are, for example, the means in which the cleave energy is reduced to allow the controlled creation and propagation of a cleave front. The cleave front is in itself a higher stress region and once created, its propagation requires a lower energy to further cleave the material from this initial region of fracture. The energy required to propagate the cleave front is called the cleave front propagation energy (Ep). The relationship can be expressed as:
Ec=Ep+[cleave front stress energy]
A controlled cleaving process is realized by reducing Ep along a favored direction(s) above all others and limiting the available energy to be below the Ep of other undesired directions. In any cleave process, a better cleave surface finish occurs when the cleave process occurs through only one expanding cleave front, although multiple cleave fronts do work.
This technique uses a relatively low temperature during the controlled cleaving process of the thin film to reduce temperature excursions of the separated film, donor substrate, or multi-material films according to other embodiments. This lower temperature approach allows for more material and process latitude such as, for example, cleaving and bonding of materials having substantially different thermal expansion coefficients. In other embodiments, the present invention limits energy or stress in the substrate to a value below a cleave initiation energy, which generally removes a possibility of creating random cleave initiation sites or fronts. This reduces cleave damage (e.g., pits, crystalline defects, breakage, cracks, steps, voids, excessive roughness) often caused in pre-existing techniques. Moreover, the present invention reduces damage caused by higher than necessary stress or pressure effects and nucleation sites caused by the energetic particles as compared to pre-existing techniques.
The present invention achieves these benefits and others in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.